Microlithographic projection exposure apparatuses are used, in particular, when producing integrated circuits or other microstructured or nanostructured components and serve to image a pattern of a mask or a reticle onto a photosensitive layer of a substrate. To this end, a conventional projection exposure apparatus contains a light source and an illumination system, which prepares electromagnetic radiation emitted by the light source and directs it onto the pattern. A portion of the pattern illuminated by the illumination system is imaged onto the photosensitive layer of a substrate using a projection lens of the projection exposure apparatus. In general, so-called wafers made out of a semiconductor material are used as substrate.
The progressive miniaturization of the structures of semiconductor components and the desire for faster production processes with shorter exposure times lead to ever higher expectations on the imaging properties of the projection exposure apparatuses and, in particular, of the projection lenses. The pattern should be imaged onto the photosensitive layer with imaging aberrations that are as small as possible during the whole period of operation of the projection exposure apparatus.
In addition to imaging aberrations due to manufacturing and assembly tolerances, imaging aberrations only occurring during operation are also known. Thus, ageing effects, e.g. a compaction of the material, and hence a locally delimited change in form may occur in optical elements at locations which are exposed to a particularly high light intensity over a relatively long time. A further cause for imaging aberrations due to operation lies in the absorption in the optical elements of the projection lens of part of the electromagnetic radiation used for the exposure. The power absorbed in the process leads to inhomogeneous heating of the optical elements, as a result of which there are changes in the refractive index, expansions and mechanical tensions. There are aberrations of the wavefront propagating in the projection lens as a result of this effect, which is referred to as “lens heating”. Lens heating represents an increasing issue in semiconductor lithography due to the desired increase in power of the employed electromagnetic radiation and the advancing miniaturization.
Since the option of dynamic correction of imaging aberrations occurring or changing during operation is becoming ever more important, modern projection exposure apparatuses contain a multiplicity of optical manipulators. In these optical manipulators, the optical effect can be modified in a specific manner during the operation by way of appropriate actuators. Depending on the measured or extrapolated wavefront error, a wavefront deformation can be induced by the manipulators during the operation, which wavefront deformation is at least partly suitable for compensating the currently occurring wavefront error.
Various microlithographic optical manipulators which have a multiplicity of zones, distributed over the cross section of the beam path, with an individually adjustable optical effect are known. By way of example, US 2008/0204682 A1 describes a manipulator which applies infrared radiation onto an optical element in the form of a lens element. In the process, the lens element is irradiated variably in two dimensions. As a result of the absorption of the infrared radiation there is a corresponding heating of the irradiated lens element sections.
Furthermore, adaptive mirrors as manipulators for a microlithographic projection exposure apparatus are known. By way of example, DE 102011081603 A1 illustrates a mirror with a piezoelectric layer and a reflecting coating arranged thereabove. A different local deformation of the reflecting coating can be brought about, depending on the applied voltage, by way of a multiplicity of control electrodes distributed over the piezoelectric layer. A mirror configured in this way is also described in WO 2011/074319. Furthermore, e.g. JP 2013-161992 A or JP 2013-106014 A have disclosed adaptive mirrors, in which actuators in the form of piezoactuators or ultrasonic motors contact at various points at the rear side of the mirror. Therefore, the mirror can be conceived as distributed into zones that are actuated individually or in combination.
Furthermore, WO 2008/034636A2 describes a current-operated thermal manipulator with a plane parallel quartz plate. The plate contains a two-dimensional matrix of heating zones, which can be heated individually by way of conductor tracks and ohmic structures. By adjusting the introduced electric power, it is possible to set an individual temperature and hence a specific refractive index for each zone.
For the purposes of compensating wavefront errors which occur, or change, during the operation of a projection exposure apparatus, each zone of the described manipulators are actuated by a suitable travel command in such a way that, overall, a correction that is as ideal as possible is obtained. Here, adjustment options of other manipulators of the projection exposure apparatus are to be taken into account when determining the travels. Moreover, a number of boundary conditions are observed. By way of example, the travel for one zone can also influence adjacent zones or restrict the travels thereof. Moreover, the thermal neutrality over all zones are maintained in the case of thermal manipulators in order to avoid an impairment of adjacent structures. Determining the travels for compensating a measured wavefront error therefore leads to a very complicated optimization problem which, in general, can no longer be solved in real time during the operation of the projection exposure apparatus.
Therefore, conventionally, the travels of all zones and also of the other manipulators provided in the projection exposure apparatus are calculated in advance for specific wavefront errors, for example for specific Zernike coefficients, and provided as travel vectors in a memory for a control unit of the projection exposure apparatus. The control unit subsequently generates a travel vector with travels for all zones, suitable for compensating the wavefront error for a measured or extrapolated wavefront error, with the aid of the stored travel vectors during operation.
A disadvantage of this procedure is that the previously calculated and provided travel vectors assume a manipulator with a travel characteristic within a specific target specification. If a travel characteristic which deviates from the target specification in one or more zones arises during the operation as a result of a fault, it cannot be taken into account when generating a travel vector. Therefore, a wavefront error is no longer corrected in an ideal manner. Since a new calculation of all provided travel vectors which takes the deviating travel characteristic into account is too time-consuming, the defective components are ultimately be replaced.
An example of such a fault is an electric short circuit between two adjacent zones of a thermal manipulator, as a result of which both zones are always operated with the same electric power. Furthermore, optical elements, e.g. deformable mirrors or heatable plates, in which the travel characteristic of one or more zones already deviates too strongly from the target specifications after being manufactured, cannot be used for a manipulator.
A further example of the aforementioned fault is the failure of a channel of a manipulator which applies infrared light onto an optical element, with the consequence that the correction capability of the manipulator collapses locally. Furthermore, mechanical components which exert pressure or tension onto an adaptive mirror may be affected by a failure. It is likewise possible, for example, for the contact of an adaptive mirror with a piezoelectric layer to be damaged such that individual actuatable zones can no longer be actuated.
A further problem emerges by virtue of it being possible that a predetermined travel range of one or more travels is exceeded when generating travel commands. These travels could subsequently no longer be implemented in their entirety. Such clipping due to overdriving would lead to a worse compensation of the wavefront errors that are present or to the generation of further wavefront errors. Moreover, the control command generation overall could become unstable. Since the known control methods with provided travel vectors only allow an insufficient reaction to threatened or occurring clipping, the manipulators will not be driven up to the range boundaries from the outset. As a result of this, use is disadvantageously not made of the complete capability of the manipulators.